Devices, methods, and systems for uplink synchronization in time division multiple access (TDMA) satellite network

ABSTRACT

Devices, methods, and systems for uplink synchronization in time division multiple access (TDMA) satellite network. In one embodiment, an earth-based satellite terminal is configured to communicate with a satellite hub through a satellite using the TDMA communication protocol. The earth-based satellite terminal is configured to determine its own location, a location of the satellite, estimate a distance between the location of the terminal and the location of the satellite, determine a Coarse Timing Advance based on the distance that is estimated, and transmit data to the satellite based on the Coarse Timing Advance and the TDMA communication protocol. The Coarse Timing Advance may allow uplink TDMA communication without a preamble transmission on a random access channel, the preamble transmission being required in many conventional systems.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/667,940, filed on May 7, 2018, the entire content of which is hereinincorporated by reference.

FIELD

The present disclosure relates generally to wireless communicationsystems. More specifically, the present disclosure relates to a wirelesscommunication system with time division multiple access (referred to as“TDMA”) on the uplink (also referred to as the “return link”).

BACKGROUND

The physical layer of a satellite air interface may include TimeDivision Multiple Access (TDMA) on the Uplink (UL), also referred to asthe Return Link (RL). TDMA may be present as one component of themultiple access scheme of air interface, which may include othercomponents, such as Frequency Division Multiple Access (FDMA). OFDMA isan example of such mixed, multiple access schemes. The teachings of thisinvention are therefore applicable to modern air interfaces using OFDMA,such as LTE and 5G, as they are to classic air interfaces such as GSMwhich used exclusively TDMA.

In TDMA, different users' signals are kept separated, that is mutuallynon-interfering or orthogonal, based on distinct, non-overlappingarrival times at the satellite. In a satellite network, compared to aterrestrial wireless network, user terminals may be distributed over amuch larger geographical area, involving much greater differentialpropagation delays among the earth-based satellite terminals than in aterrestrial wireless network. This increases the challenge ofmaintaining uplink orthogonality in satellite networks relative toterrestrial networks. In TDMA systems, the uplink orthogonality ismaintained by time staggering the uplink transmissions from differentterminals, by a technique known as “timing advance” (also referred toherein as “TA”).

SUMMARY

Conventional schemes have problems when the values of the requiredtiming advance exceed a frame duration of the air interface in use. Inthe present context, a “frame” is a period within the TDM/TDMA systemtime structure which, if exceeded by the required timing advance,creates protocol specification issues that may lead to overlap betweentransmissions from different terminal transmissions. The systems andmethods of the present disclosure solve the problems of the conventionalschemes.

For example, in one embodiment, the present disclosure includesearth-based satellite terminal including a satellite transceiver, amemory, and an electronic processor communicatively connected to thememory and the satellite transceiver. The satellite transceiver isconfigured to communicate with a satellite using a time divisionmultiple access (TDMA) communication protocol. The electronic processoris configured to determine a location of the earth-based satelliteterminal, determine a location of the satellite, estimate a distancebetween the location of the earth-based satellite terminal and thelocation of the satellite, determine a Coarse Timing Advance based onthe distance that is estimated, and control the satellite transceiver totransmit data to the satellite based on the Coarse Timing Advance andthe TDMA communication protocol.

For example, in a second embodiment, the present disclosure includes awireless communication method. The method includes determining, with anelectronic processor of an earth-based satellite terminal, a location ofthe earth-based satellite terminal. The method includes determining,with the electronic processor, a location of a satellite. The methodincludes estimating, with the electronic processor, a distance betweenthe location of the earth-based satellite terminal and the location ofthe satellite. The method includes determining, with the electronicprocessor, a Coarse Timing Advance based on the distance that isestimated. The method also includes controlling, with the electronicprocessor, a satellite transceiver of the earth-based satellite terminalto transmit data to the satellite based on the Coarse Timing Advance anda time division multiple access (TDMA) communication protocol.

In a third embodiment, the present disclosure includes a time divisionmultiple access (TDMA) communication system including a satellite and anearth-based satellite terminal. The earth-based satellite terminalincluding a satellite transceiver, a memory, and an electronicprocessor. The satellite transceiver is configured to communicate withthe satellite using the TDMA communication protocol. The electronicprocessor is configured to determine a location of the earth-basedsatellite terminal, determine a location of the satellite, estimate adistance between the location of the earth-based satellite terminal andthe location of the satellite, determine a Coarse Timing Advance basedon the distance that is estimated, and control the satellite transceiverto transmit data to the satellite based on the Coarse Timing Advance andthe TDMA communication protocol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates a terrestrial wirelesscommunication system.

FIG. 2 is a diagram that illustrates a satellite wireless communicationsystem.

FIG. 3 is a block diagram that illustrates an earth-based satelliteterminal, according to various exemplary embodiments of the presentdisclosure.

FIG. 4 is a timing diagram that illustrates a timing advance by theearth-based satellite terminal of FIG. 3 that overcomes the differentialpropagation delay in the satellite wireless communication system of FIG.2.

FIGS. 5-7 are diagrams that illustrate examples of differential (roundtrip) propagation delays involving a satellite with a global beam, 183beams, and 21 beams, respectively.

FIG. 8 is a diagram that illustrates a figure-of-eight pattern of thesatellite of FIG. 2.

FIG. 9 is a flowchart of a wireless communication method, according tovarious exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before any embodiments of the present disclosure are explained indetail, it is to be understood that the present disclosure is notlimited in its application to the details of construction and thearrangement of components set forth in the following description orillustrated in the following drawings. The present disclosure is capableof other embodiments and of being practiced or of being carried out invarious ways.

FIG. 1 is a diagram that illustrates a terrestrial wirelesscommunication system 100. In the example of FIG. 1, the terrestrialwireless communication system 100 includes a base station tower 102, afirst terrestrial terminal 104, a second terrestrial terminal 106, andthree cells 108-112. As illustrated in FIG. 1, the first terrestrialterminal 104 is located a first distance (“d1”) from the base stationtower 102 and the second terrestrial terminal 106 is located a seconddistance (“d2”) from the base station tower 102. The first terrestrialterminal 104 and the second terrestrial terminal 106 are located in thecell 108.

The “propagation delay” is the roundtrip delay from the base stationtower 102 to one of the two different terrestrial satellite terminals104 and 106. The differential, roundtrip propagation delay between thetwo different earth-based satellite terminals 104 and 106 is defined byEquation 1.Differential,roundtrip propagation delay=2*(d1−d2)  (1)

FIG. 2 is a diagram of a satellite wireless communication system 200. Inthe example of FIG. 2, the satellite wireless communication system 200includes a satellite 202, a first earth-based satellite terminal 204, asecond earth-based satellite terminal 206, and three spotbeams 208-212,and a satellite hub 214. As illustrated in FIG. 2, the first earth-basedsatellite terminal 204 is located a first distance (“d1”) from thesatellite 202 and the second earth-based satellite terminal 206 islocated a second distance (“d2”) from the satellite 202. The firstearth-based satellite terminal 204 and the second earth-based satelliteterminal 206 are located in the spotbeam 208.

In the example of FIG. 2, the satellite 202 is a bent-pipe, ornon-demodulating satellite. In the satellite wireless communicationsystem 200, end-to-end communication occurs between the earth-basedsatellite terminals 204 and 206 and the satellite hub 214 (also referredto as a “satellite gateway”).

The satellite hub 214 includes one or more baseband processors 216,radio equipment 218, and an earth station antenna 220. The basebandprocessors 216 assess the uplink timing advance. The earth stationantenna 220 communicates with the satellite 202 via a feeder link 222.

In some examples, where the satellite 202 is a demodulating satellite,the satellite 202 may be perform the assessment of the uplink timingadvance on board. The teachings of the present disclosure are equallyapplicable to both cases because the additional propagation delay of thefeeder link 222, in the case of bent-pipe satellites, makes nocontribution to the differential delay between terminals. Equalizingthis differential delay is the main objective of uplink synchronizationin a TDMA system.

The “propagation delay” is the roundtrip delay from the satellite 202 toeach of the two different earth-based satellite terminals 204 and 206.The differential, roundtrip propagation delay between the two differentearth-based satellite terminals 204 and 206 is also defined byEquation 1. The differential, roundtrip propagation delay in thesatellite wireless communication system 200 is likely to be larger thanthe differential, roundtrip propagation delay in the terrestrialwireless communication system 100 because the first distance d1 and thesecond distance d2 of FIG. 2 are larger than the first distance d1 andthe second distance d2 of FIG. 1, respectively.

The greater differential propagation delay in a satellite network (thatis, the propagation delay difference between the nearest and farthestuser terminal location in a given spotbeam, relative to the satellite)makes it more difficult to maintain the time orthogonality on the uplinkthan the terrestrial network, for TDMA based communication protocols.Therefore, there is the possibility of uplink bursts colliding at thesatellite unless the transmit times of the bursts are appropriatelystaggered in time to avoid collision.

In order to appreciate the improvements afforded by the presentinvention and introduce relevant terms of reference, it is worthwhilereviewing the technical approach used in GMR-2, a satellite airinterface based on GSM, which also uses TDMA. The technical approach ofGMR-2 is described below in greater detail.

GMR-2 is an ETSI and TIA standard and is representative of aconventional TDMA communication system. GMR-2 uses a timing advanceparameter that comprises a fixed part and two variable parts. The fixedpart (referred to as a Coarse Timing Advance or “CTA”) is linked to agiven spotbeam. The variable parts (referred to as Fine Timing Advanceor “FTA” and a Delta Timing Advance or “DTA”) are linked, respectively,to (a) a location of a given earth-based satellite terminal within thespotbeam and (b) the movements of the given earth-based satelliteterminal and the satellite 202. The FTA and DTA variable parts arecommunicated to the given earth-based satellite terminal by thesatellite hub 214 through the satellite 202 via one or more downlinkmessages.

The timing advance estimations by the satellite hub 214 are based onobserved arrival times of uplink random access channel (RACH) preamblebursts at the satellite, relative to their ideal arrival time accordingto the “system TDM.” The “system TDM” defines the system time base forthe network and is referenced to the location of the satellite. Theideal arrival time is at the center of the RACH window 408 in the systemTDM frame structure, illustrated as frame 402A in FIG. 4. Unlike the DTAand FTA, the CTA does not have to be communicated to the givenearth-based satellite terminal because the given earth-based satelliteterminal determines the CTA upon identifying the spotbeam to which thegiven earth-based satellite terminal has established access, alsoreferred to as “camping on.” The identification of the spotbeam may befrom information that is directly loaded onto the given earth-basedsatellite terminal via a configuration process or distributed to thegiven earth-based satellite terminal via a downlink (DL) controlchannel, such as a broadcast control channel (BCCH).

To initiate a new inbound call in GMR-2, a given earth-based satelliteterminal synchronizes to the downlink TDM, determines the spotbeam ID,and the CTA based on the spotbeam ID, and then sends an uplink RACHpreamble burst with the said CTA. This RACH preamble burst arrives atthe satellite 202 with an error, τ, relative to the center of the RACHwindow in the system TDM. The satellite hub 214 determines the value ofti and communicates this to the given earth-based satellite terminal asFTA. The given earth-based satellite terminal adds FTA to CTA and makesthat the timing advance for all subsequent uplink transmissions.Periodically, as the given earth-based satellite terminal changeslocation and the satellite 202 moves, further adjustment to the timingadvance may be necessary, and are made with the DTA. It is noteworthythat FTA and DTA are required because, while CTA may be sufficient toensure that the error in RACH burst may be sufficiently small to ensurethat it lands inside the RACH window (with its allowed guard time), thiserror with respect to the RACH burst would be too large for the uplinkTDMA traffic and control channels. To decrease the error, the uplinksynchronization process includes the steps of: (1) the hub 214 making anassessment of the time-of-arrival error in the RACH burst andcommunicating this error to the earth-based satellite terminals throughthe FTA, (2) the hub 214 continuously assessing the errors in thearrival times of the uplink TDMA bursts, and (3) providing feedback tothe earth-based satellite terminals of the said timing errors throughthe DTA, if the said additional correction is required.

Like GMR-2, in some examples, a system of the present disclosure alsouses a hierarchy of CTA, FTA, and DTA. However, unlike conventional TDMAcommunication systems (e.g., GMR-2), the CTA of the present disclosureis based on a direct estimation of the distance from the earth-basedsatellite terminal to the satellite. This direct estimation may be madewith sufficient accuracy that the size of the of the RACH window may bereduced to a range (few milliseconds) that is similar to that used inmodern cellular wireless systems. This direct estimation alsofacilitates alignment between cellular and satellite ecosystems, whichis extremely advantageous for the satellite ecosystems. Furthermore, thelinkage between spotbeam size and RACH window size, present inconventional TDMA communication systems, is severed in the presentsystem. Thus, large, global beams, which have advantages in certainapplications, may be deployed with small RACH windows, i.e., a fewmillisecond as discussed above. The available of large, global beamswith small RACH windows enables beam design flexibility that does notexist in the conventional TDMA systems.

In some embodiments, depending on the accuracy with which the distancefrom the earth-based satellite terminal to the satellite is estimated,the CTA of the present disclosure may, by itself, provides sufficientaccuracy for uplink synchronization of all TDMA channels. A CTA thatprovides sufficient accuracy for uplink synchronization of all TDMAchannels would reduce the transaction latency and increase uplinkcapacity by reducing or eliminating the need for a RACH preamble.

FIG. 3 is a block diagram illustrating an earth-based satellite terminal300 in accordance with various embodiments of the present disclosure. Inthe example of FIG. 3, the earth-based satellite terminal 300 includes amemory 302, an electronic processor 304 (for example, a microprocessoror another suitable processing device), a satellite transceiver 306, andan input/output (I/O) interface 308.

It should be understood that, in some embodiments, the earth-basedsatellite terminal 300 may include fewer or additional components inconfigurations different from that illustrated in FIG. 3. Also, theearth-based satellite terminal 300 may perform additional functionalitythan the functionality described herein. As illustrated in FIG. 3, thememory 302, the electronic processor 304, the satellite transceiver, andthe I/O interface 308 are electrically coupled by one or more control ordata buses enabling communication between the components.

The memory 302 (also referred to as a “non-transitory computer-readablemedium”) may include a program storage area (for example, read onlymemory (ROM)) and a data storage area (for example, random access memory(RAM), and other non-transitory, machine-readable medium). In someexamples, the program storage area stores the instructions regarding theTiming Advance program 310.

The electronic processor 304 executes machine-readable instructionsstored in the memory 302. For example, the electronic processor 304executes instructions stored in the memory 302 to perform the timingadvance functionality described below regarding uplink synchronizationat the satellite 202. In some examples, the electronic processor 304 amicroprocessor, an application-specific integrated circuit (“ASIC”), orother suitable electronic processor. In one example, the Timing Advanceprogram 310 causes the electronic processor 304 to directly estimate thedistance from the earth-based satellite terminal 300 to the satellite202, determine a CTA from the direct estimation, and control the RACHtransmission based on the CTA. The specific methods performed by theelectronic processor 304 by executing the Timing Advance program 310 isexplained in greater detail below with respect to FIGS. 9-10.

In some examples, the I/O interface 308 may include an Ethernet I/Ointerface. In other examples, the I/O interface 308 may include awireless interface (for example, WiFi, LTE, LTE Advanced, 5G, or othersuitable wireless interface). In yet other examples, the I/O interface308 may include a navigation transceiver (for example, a GPS transceiverand/or a GNSS transceiver). In some examples, the I/O interface 308 mayinclude a combination of an Ethernet I/O interface, a wirelessinterface, and/or the navigation transceiver.

FIG. 4 is a timing diagram that illustrates a timing advance by theearth-based satellite terminal 300 of FIG. 3 that overcomes thedifferential propagation delay in the satellite wireless communicationsystem of FIG. 2. In the example of FIG. 4, the timing diagram 400includes three timing pairs 402-406 for transmission and reception ofdata sequences by either the satellite 202 and the earth-based satelliteterminal 204 or the satellite 202 and the earth-based satellite terminal300.

As explained above, for ease of understanding, the satellite 202 is abent-pipe, or non-demodulating satellite. Therefore, the ultimate sourceand destination of signals said to be “transmitted” and “received” bythe satellite 202 is the satellite hub 214, which is communicativelycoupled to the satellite 202 by the feeder link 222. The estimation ofpropagation delays is performed at the satellite hub 214. As thepropagation delay of the feeder link 222 is common between differentterminals, and do not contribute to the differential delays, thepropagation delay of the feeder link 222 is not addressed in thisdisclosure. Additionally, as explained above, the concepts and methodstaught herein are equally applicable to both non-demodulating satellitesas well as demodulating satellites.

As illustrated in FIG. 4, the first timing pair 402 illustrates a TimeDivision Multiplex (TDM) frame structure 402A for the satellite network.The TDM frame structure 402A is considered the network-wide, referencesystem time, or “system TDM frame.” The frame structure includes timeslots reserved for both downlink and uplink data sequences, or TDMchannels. One of the uplink TDM channels is reserved for the receptionof a Random Access Channel (RACH), transmitted by the earth-basedsatellite (EBST) terminals 204, 206 of FIG. 2 or the earth-basedsatellite terminal (EBST) 300 of FIG. 3. A downlink burst transmittedaccording to the system TDM frame 402A is received by the earth-basedsatellite terminal 204 or the earth-based satellite terminal 300 with aone hop delay, ΔT, and is shown by the timing 402B. With respect to theearth-based satellite terminal 204, the received system TDM frame 402Brepresents the local system time for the earth-based satellite terminal204, and is linked to its location.

The second timing pair 404 in FIG. 4 illustrates an example where notiming advance is used by the earth-based satellite terminal 204.According to this approach, the uplink RACH burst (for messages such asaccess requests) is transmitted at its designated time slot in the TDMframe 404A, which is aligned with the received system TDM frame 402B.The RACH burst in the TDM frame 404A burst is received at the satelliteas the TDM frame 404B with a one hop delay of ΔT. As the TDM frame 404Aitself includes a one hop delay of ΔT referenced to the system TDM frame402A, there is a net timing error, or offset, of 2·ΔT in the RACH burstreceived at the satellite 202, relative to its expected position at thecenter of the RACH window 408 in the system TDM frame 402A.

As illustrated in FIG. 4, the third timing pair 406 mitigates the timemisalignment in the second timing pair 404 by advancing the transmittime of the earth-based satellite terminal 300 relative to its localsystem time 402B by 2·ΔT. The data sequence associated with the timing406A is an example of such a time advanced transmission. This datasequence is received at the satellite 202 as the data sequenceassociated with the timing 406B, which is exactly time aligned with thereference system time, represented by the timing 402A. Stateddifferently, the RACH burst by the earth-based satellite terminal 300 isreceived at the satellite 202 in the RACH window 408.

As mentioned above, the conventional methods face problems withdifferential propagation delay when a satellite spotbeam grows largerand/or the frame size becomes smaller. The conventional solution is touse larger frames, which can accommodate larger RACH windows, tomitigate the above problems. The term “larger” means that the size ofthe RACH window is increased enough to accommodate the large delayspread (with guard time inside the frame) when a satellite spotbeam isincreased in size. As described in greater detail below for thesatellite spotbeam examples provided in FIGS. 5-7, the size of the RACHwindow would need to be sufficient to accommodate a propagation delayrange of at least 1.3 milliseconds (ms) for 183 spotbeams and 34 ms fora global spot beam.

However, the use of large RACH windows, and consequently, larger frames,has certain disadvantages. Satellite ecosystems may benefit by reusingcomponents developed for terrestrial wireless ecosystems owing to thegreater scale of the terrestrial wireless ecosystems, which can supportgreater R&D expenditures. However, to adapt modern terrestrial wirelessair interfaces, for example, LTE Advanced and 5G, for use in mobilesatellite applications, the frame size needs to be similar to those usedin terrestrial wireless air interfaces, i.e., approximately 1 ms.Therefore, it is desirable to adopt such small frames even in satellitesystems, although, the long round trip delays, especially forconventional GEO satellites, demand larger frames, of the order of tensof milliseconds.

Uplink time staggering (referred to as Timing Advance or “TA”) is alsoperformed in existing terrestrial wireless networks but the requiredtiming advance is small compared to the uplink frame duration. When therequired timing advance exceeds the uplink frame duration, TDMAcommunication protocol design problems may result.

The desired beam sizes of satellite networks range from having roundtrip delays of a few milliseconds to tens of milliseconds. FIGS. 5-7illustrate examples of differential (round trip) propagation delaysinvolving Ligado Network's SkyTerra-1 satellite (for example, thesatellite 202 of FIG. 2) for various, example beam sizes. As illustratedin FIG. 5, the satellite 202 has a differential propagation delay of 34ms to cover an area involving the US and Southern Canada with a single“global beam.” As illustrated in FIG. 6, the satellite 202 has adifferential propagation delay that varies from 1.3 ms to 12.4 ms when adeployment of 183 beams covers the same area (wherein a beam isapproximately 150 km in diameter). As illustrated in FIG. 7, thesatellite 202 has a differential propagation delay that varies from 5.9ms to 12.6 ms (ignoring the Hawaii beam) with an intermediate beam sizecorresponding to 21 beams and covering the US and Canada.

The frame size in 3GPP wireless terrestrial air interfaces isapproximately 1 ms (LTE-M, NB IOT). None of the above beam designs wouldmeet the requirement of limiting timing advance to less than the framesize of 1 ms.

As explained above, the earth-based satellite terminal 300 overcomes thechallenge of the small frame size in 3GPP wireless terrestrial airinterfaces by determining a Coarse Timing Advance (CTA) based on anestimated distance between the earth-based satellite terminal 300 andthe satellite 202. The earth-based satellite terminal 300 may determineits own location autonomously using a navigation service (for example,GPS or GNSS), a terrestrial wireless navigation system, an inertialnavigation system, or some combination thereof. Additionally, theearth-based satellite terminal 300 may acquire information indicative ofthe location of the satellite through one of a variety of means. Forexample, the earth-based satellite terminal 300 may acquire theephemeris data of the satellite 202, i.e., the absolute location of thesatellite 202 in space, via a broadcast over a downlink control channel(for example, a broadcast control channel or “BCCH”), wherein theephemeris information is gathered by ancillary equipment at thesatellite hub 214, which periodically estimates the location of thesatellite in space through ranging methods known in the prior art.

Before sending messages on the uplink to the satellite, the earth-basedsatellite terminal 300 synchronizes to the time/frequency references asobserved at the location of the earth-based satellite terminal 300.After the time/frequency synchronization, the earth-based satelliteterminal 300 transmits a connection request message on an uplink TDMARACH channel, with a timing advance that is designed to be synchronizedto the uplink random access time window, or slot, in the system TDM,which is referenced to the location of the satellite 202, as shown inFIG. 4 and explained above. The timing advance is defined by Equation 2.Timing Advance=2*ΔT=2*(D/c)  (2)

In Equation 2, ΔT is the one-way propagation time from the satellite tothe earth-based satellite terminal, D is the distance from the satellite202 to the earth-based satellite terminal 300, and c is the velocity oflight. In Equation 2, it is assumed that propagation is essentially overfree space, although passage through the atmosphere (ionosphere andtroposphere) will cause some additional delay, which may be upperbounded to 10 meters (m), or 10/3E8 s=33 nanoseconds (ns). Therefore,the propagation delay may safely be assumed to correspond to free spacepropagation.

Additionally, errors in the location of the earth-based satelliteterminal 300 and/or the location of the satellite 202 contribute to anerror in the timing advance. However, modern GPS/GNSS modules in theearth-based satellite terminal 300 have an accuracy that is better than10 m which, as described above, leads to an error of only 33 (ns), whichis negligible compared to the frame length of 1 ms. The location of thesatellite 202 is measured continuously by the satellite networkinfrastructure and is known with an accuracy that is sufficient to limitthe error contribution to a negligible value, relative to RACH receivewindow of 1 ms.

The satellite ephemeris data may be broadcast to the earth-basedsatellite terminal 300 on a downlink control channel using severaldifferent approaches, which consume different amount of valuablesatellite network capacity. In a first approach, the absolute location,e.g. the location specified by latitude, longitude and distance from thecenter of the Earth may be broadcast periodically on the broadcastcontrol channel. However, this broadcast would require a large amount ofinformation and more frequent broadcast than the other approachesdescribed below, consuming relatively more downlink capacity.

In a second approach, the earth-based satellite terminal 300 haspredefined information about the approximate location of the satellitein space. By moving the origin of the coordinate system from the centerof the Earth to a fixed, or quasi-fixed, point in space in the vicinityof the satellite, the amount of information that needs to be transmittedfrom the hub to the terminal is substantially reduced. This is becausethe amount of the said information is proportional to the length of theposition vector from the new coordinate system origin to the actuallocation of the satellite. One candidate for the new origin may be themean position of the satellite over a long period, such as a year orseveral months, after which the location of the origin may be updatedvia the BCCH.

In a third approach, further reduction in the forward link capacityrequired to update ephemeris information may take the following form. Asatellite's dynamics in space typically has a relatively well-known andpredictable pattern. This pattern is often referred to as a ‘figure ofeight’, although the figure is in three dimensions. If we assume that weknow the approximate location of the satellite as function of time, thisinformation may be stored in the memory of the terminal as the newcoordinate origin, albeit as a function of time. Thereby, the size ofthe position vector of the true location of the satellite relative tothis new, dynamic but known origin will be further reduced relative tothe second approach, where the origin was fixed in space. The positionvector representing the actual location of the satellite relative to theorigin, be it fixed or dynamic, is referred to as an error vector as itrepresents the error of the assumed location of the satellite (thecoordinate origin, or the modeled location) relative to the actuallocation.

The repetitive, figure-of-eight pattern traced out by the satellite 202in space, assuming the satellite is of the geosynchronous orbit type, isillustrated in FIG. 8. This may be viewed as a mathematical model of themean position of the satellite 202 in space, derived from theoreticalconsiderations and informed by empirical observations. At a selectedfrequency, for example, once every few seconds (s), the satellite hub214 broadcasts the error (a three-dimensional position-error vector)between the present observed and the modeled position of the satellite,as illustrated in FIG. 8. The earth-based satellite terminal 300determines the location of the satellite 202 using the broadcast errorvector.

In a fourth approach, the satellite hub 214 does not broadcastinformation about the satellite's position. Instead, thefigure-of-eight, or any other, theoretical model of the satellite's meanposition (as a function of time) is treated as the true position of thesatellite 202 and is used to determine the timing advance. This approacheliminates consumption of downlink satellite capacity due to continuousephemeris distribution—the satellite location is determined unilaterallyby the terminal with no assistance from the hub. However, this does notrule out the possibility of infrequent updates of the modeled satellitelocation—whether the model is updated at all or never becomes a capacityoptimization question. The potential for uplink timing error will begreater in this case than in the other approaches but may be acceptablewhen the satellite dynamics are low, e.g. when satellite station keepingis employed, or relatively long RACH windows are acceptable.

The model of the satellite's position may be characterized by a limitedset of key parameters (for examples, maximum azimuth and elevationexcursions), involving less information content than a point-by-pointmap of the figure-of-eight pattern. The key parameters may be broadcastto all terminals. This model update is necessary because the maximumexcursions of the figure-of-eight pattern changes over the life of thesatellite.

The above approaches are not limited to geostationary satellites asdescribed above. Indeed, they are also applicable to other satellitetypes (MEO/LEO) where the dynamics of the satellite orbits aresufficiently well known.

In some cases, where GPS/GNSS modules in the earth-based satelliteterminal 300 may experience cold start or lack of GPS satelliteacquisition, the earth-based satellite terminal 300 may use its lastposition information in the memory to estimate a Coarse Timing Advance,which can be subsequently improved by Fine Timing Advance and DeltaTiming Advance.

Although there are similarities with conventional systems in the use ofFTA and DTA to optimize and track uplink timing accuracy, the primeadvantage of the present disclosure is a superior CTA. A CTA based onthe satellite ephemeris makes the system of the present disclosureindependent of satellite spotbeam size and conducive to small TDM framesizes without any significant changes in the baseline terrestrial airinterface protocols.

In some embodiments, given favorable satellite dynamics, the methodsdescribed herein for determining CTA may be sufficiently accurate thatFTA and DTA may be redundant. In other words, the timing accuracyafforded by the CTA of the present disclosure may be sufficient fordirect uplink transmission for traffic and control channels withoutfeedback from a satellite hub (e.g., the satellite hub 214).

While the present disclosure describes sending a three dimensional errorvector from the satellite hub 214 to the earth-based satellite terminal300 through satellite 202, the methods herein are not dependent onsatellite transport of the three dimensional error vector. For example,where coverage is available, the three dimensional error vector may besent by terrestrial means, such as cellular public landmobile networks(PLMN) or wireless local area networks (WLAN), such as Wi-Fi coupled tothe internet. Terrestrial transport of the three dimensional errorvector may conserve precious satellite capacity. Where, terrestrialcoverage is insufficiently ubiquitous, multimode (satellite-terrestrial)transport may be used. Exclusively terrestrial coverage may be used whenthe earth-based satellite terminal 300 is mobile and the terrestrialcoverage blockages are short relative to the time rate of change of thethree dimensional error vector.

FIG. 9 is a flowchart of a wireless communication method 900. FIG. 9 isdescribed with reference to the earth-based satellite terminal 300 ofFIG. 3.

The method 900 includes determining, with an electronic processor 304, alocation of the earth-based satellite terminal 300 (at block 902). Forexample, determining the location of the earth-based satellite terminalfurther includes receiving navigation information from a navigationtransceiver, a terrestrial wireless navigation system, an inertialnavigation system, or a combination thereof via an input/outputinterface of the earth-based satellite terminal, and determining thelocation of the earth-based satellite terminal based on the navigationinformation

The method 900 includes determining, with the electronic processor 304,a location of a satellite (at block 904). In some examples, determiningthe location of the earth-based satellite terminal further includessetting a last known location of the earth-based satellite terminal asthe location of the earth-based satellite terminal.

In other examples, determining the location of the satellite furtherincludes periodically receiving an absolute location of the satellitefrom a satellite hub associated with the satellite, the absolutelocation specifying a latitude, a longitude, and a distance from thecenter of the Earth with respect to the satellite. In these otherexamples, periodically receiving the absolute location of the satellitefrom the satellite hub may further include periodically receiving theabsolute location of the satellite from the satellite hub via thesatellite transceiver and the satellite. Additionally or alternatively,in these other examples, periodically receiving the absolute location ofthe satellite from the satellite hub may further include periodicallyreceiving the absolute location of the satellite from the satellite hubvia a wireless interface of the earth-based satellite terminal and aterrestrial communication network.

In yet other examples, determining the location of the satellite mayfurther include retrieving predefined information that is stored in amemory and represents a model of an approximate position of thesatellite, periodically receiving a three dimensional error vector froma satellite hub, the three dimensional error vector representing anerror between a present location of the satellite and a modeled positionof the satellite, and determining the location of the satellite based onthe three dimensional error vector and the modeled position.

In some examples, determining the location of the satellite may furtherinclude retrieving predefined information that is stored in a memory andrepresents a model of an approximate position of the satellite, andsetting the location of the satellite based on the model of theapproximate position of the satellite.

In some examples, the aforementioned predefined information isindicative of a fixed point in space. In other examples, theaforementioned predefined information is indicative of a set of pointsin space that form a repetitive function of time. In these otherexamples, the repetitive function of time may represent a figure ofeight.

The method 900 includes estimating, with the electronic processor 304, adistance between the location of the earth-based satellite terminal 300and the location of the satellite 202 (at block 906).

The method 900 includes determining, with the electronic processor 304,a Coarse Timing Advance (CTA) based on the distance that is estimated(at block 908).

The method 900 also includes controlling, with the electronic processor304, a satellite transceiver to transmit data to the satellite 202 basedon the Coarse Timing Advance and the TDMA communication protocol (atblock 910). For example, the electronic processor 304 controls thesatellite transceiver to transmit a RACH burst based on the CTA and theTDMA communication protocol.

Additionally, in some examples, the method 900 may further includereceiving, with the electronic processor, a Fine Timing Advance from asatellite hub, wherein the Fine Timing Advance is determined by thesatellite hub based on an error in an arrival time of a preamble burstsent by the earth-based satellite terminal, relative to an expected,true arrival time at the satellite. In these examples, the method 900may further include controlling, with the electronic processor, thesatellite transceiver of the earth-based satellite terminal to transmitsecond data burst to the satellite based on the Coarse Timing Advance,the Fine Timing Advance, and the TDMA communication protocol.

Additionally, in some examples, the method 900 may further includereceiving, with the electronic processor, a Delta Timing Advance fromthe satellite hub, wherein the Delta Timing Advance is determined by thesatellite hub based on errors in actual arrival times of uplink trafficand control channel bursts at the satellite and sent by the earth-basedsatellite terminal according to the TDMA communication protocol,relative to their expected, true arrival times at the satellite. Inthese examples, the method 900 may further include controlling, with theelectronic processor, the satellite transceiver of the earth-basedsatellite terminal to transmit third data to the satellite based on theCoarse Timing Advance, the Fine Timing Advance, the Delta TimingAdvance, and the TDMA communication protocol.

The following are enumerated examples of the devices, methods, andsystems of the present disclosure for uplink synchronization in a timedivision multiple access (TDMA) satellite network:

Example 1: an earth-based satellite terminal comprising a satellitetransceiver configured to communicate with a satellite using a timedivision multiple access (TDMA) communication protocol, a memory, and anelectronic processor communicatively connected to the memory and thesatellite transceiver, the electronic processor configured to determinea location of the earth-based satellite terminal, determine a locationof the satellite, estimate a distance between the location of theearth-based satellite terminal and the location of the satellite,determine a Coarse Timing Advance based on the distance that isestimated, and control the satellite transceiver to transmit data to thesatellite based on the Coarse Timing Advance and the TDMA communicationprotocol.

Example 2: the earth-based satellite terminal of Example 1, furthercomprising an input/output interface including a navigation transceiver,wherein, to determine the location of the earth-based satelliteterminal, the electronic processor is configured to receive navigationinformation from the navigation transceiver, a terrestrial wirelessnavigation system, an inertial navigation system, or a combinationthereof via the input/output interface, and determine the location ofthe earth-based satellite terminal based on the navigation information.

Example 3: the earth-based satellite terminal of any of Examples 1 or 2,wherein, to determine the location of the satellite, the electronicprocessor is configured to periodically receive an absolute location ofthe satellite from a satellite hub associated with the satellite, theabsolute location specifying a latitude, a longitude, and a distancefrom the center of the Earth with respect to the satellite.

Example 4: the earth-based satellite terminal of Example 3, wherein theelectronic processor is configured periodically receive the absolutelocation of the satellite from the satellite hub via the satellitetransceiver and the satellite.

Example 5: the earth-based satellite terminal of Example 3, furthercomprising an input/output interface including a wireless interfaceconfigured to communicate with a terrestrial communication network,wherein the electronic processor is configured periodically receive theabsolute location of the satellite from the satellite hub via thewireless interface and the terrestrial communication network.

Example 6: the earth-based satellite terminal of any of Examples 1-5,wherein, to determine the location of the satellite, the electronicprocessor is configured to retrieve predefined information that isstored in the memory and represents a model of an approximate positionof the satellite, periodically receive a three dimensional error vectorfrom a satellite hub, the three dimensional error vector representing anerror between a present location of the satellite and a modeled positionof the satellite, and determine the location of the satellite based onthe three dimensional error vector and the modeled position.

Example 7: the earth-based satellite terminal of Example 6, where thepredefined information is indicative of a fixed point in space.

Example 8: the earth-based satellite terminal of Example 6, where thepredefined information is indicative of a set of points in space thatform a repetitive function of time.

Example 9: the earth-based satellite terminal of Example 8, where therepetitive function of time represents a figure of eight.

Example 10: the earth-based satellite terminal of any of Examples 1-9,wherein, to determine the location of the satellite, the electronicprocessor is configured to retrieve predefined information that isstored in the memory and represents a model of an approximate positionof the satellite, and set the location of the satellite based on themodel of the approximate position of the satellite.

Example 11: the earth-based satellite terminal of any of Examples 1-10,wherein the electronic processor is further configured to receive a FineTiming Advance from a satellite hub, wherein the Fine Timing Advance isdetermined by the satellite hub based on an error in an arrival time ofa preamble burst sent by the earth-based satellite terminal, relative toan expected, true arrival time at the satellite, and control thesatellite transceiver to transmit second data to the satellite based onthe Coarse Timing Advance, the Fine Timing Advance, and the TDMAcommunication protocol.

Example 12: the earth-based satellite terminal of Example 11, whereinthe electronic processor is further configured to receive a Delta TimingAdvance from the satellite hub, wherein the Delta Timing Advance isdetermined by the satellite hub based on errors in actual arrival timesof uplink traffic and control channel bursts at the satellite and sentby the earth-based satellite terminal according to the TDMAcommunication protocol, relative to their expected, true arrival timesat the satellite, and control the satellite transceiver to transmitthird data to the satellite based on the Coarse Timing Advance, the FineTiming Advance, the Delta Timing Advance, and the TDMA communicationprotocol.

Example 13: a wireless communication method comprising determining, withan electronic processor of an earth-based satellite terminal, a locationof the earth-based satellite terminal; determining, with the electronicprocessor, a location of a satellite; estimating, with the electronicprocessor, a distance between the location of the earth-based satelliteterminal and the location of the satellite; determining, with theelectronic processor, a Coarse Timing Advance based on the distance thatis estimated; and controlling, with the electronic processor, asatellite transceiver of the earth-based satellite terminal to transmitdata to the satellite based on the Coarse Timing Advance and a timedivision multiple access (TDMA) communication protocol.

Example 14: the wireless communication method of Example 13, whereindetermining the location of the earth-based satellite terminal furtherincludes receiving navigation information from a navigation transceiver,a terrestrial wireless navigation system, an inertial navigation system,or a combination thereof via an input/output interface of theearth-based satellite terminal; and determining the location of theearth-based satellite terminal based on the navigation information.

Example 15: the wireless communication method of any of Examples 13 and14, wherein determining the location of the earth-based satelliteterminal further includes setting a last known location of theearth-based satellite terminal as the location of the earth-basedsatellite terminal.

Example 16: the wireless communication method of any of Examples 13-15,wherein determining the location of the satellite further includesperiodically receiving an absolute location of the satellite from asatellite hub associated with the satellite, the absolute locationspecifying a latitude, a longitude, and a distance from the center ofthe Earth with respect to the satellite.

Example 17: the wireless communication method of Example 16, whereinperiodically receiving the absolute location of the satellite from thesatellite hub further includes periodically receiving the absolutelocation of the satellite from the satellite hub via the satellitetransceiver and the satellite.

Example 18: the wireless communication method of Example 16, whereinperiodically receiving the absolute location of the satellite from thesatellite hub further includes periodically receiving the absolutelocation of the satellite from the satellite hub via a wirelessinterface of the earth-based satellite terminal and a terrestrialcommunication network.

Example 19: the wireless communication method of any of Examples 13-18,wherein determining the location of the satellite further includesretrieving predefined information that is stored in a memory andrepresents a model of an approximate position of the satellite;periodically receiving a three dimensional error vector from a satellitehub, the three dimensional error vector representing an error between apresent location of the satellite and a modeled position of thesatellite; and determining the location of the satellite based on thethree dimensional error vector and the modeled position.

Example 20: the wireless communication method of any of Examples 13-19,wherein determining the location of the satellite further includesretrieving predefined information that is stored in a memory andrepresents a model of an approximate position of the satellite; andsetting the location of the satellite based on the model of theapproximate position of the satellite.

Example 21: the wireless communication method of any of Examples 13-20,further comprising receiving, with the electronic processor, a FineTiming Advance from a satellite hub, wherein the Fine Timing Advance isdetermined by the satellite hub based on an error in an arrival time ofa preamble burst sent by the earth-based satellite terminal, relative toan expected, true arrival time at the satellite; and controlling, withthe electronic processor, the satellite transceiver of the earth-basedsatellite terminal to transmit second data to the satellite based on theCoarse Timing Advance, the Fine Timing Advance, and the TDMAcommunication protocol.

Example 22: the wireless communication method of Example 21, furthercomprising receiving, with the electronic processor, a Delta TimingAdvance from the satellite hub, wherein the Delta Timing Advance isdetermined by the satellite hub based on errors in actual arrival timesof uplink traffic and control channel bursts at the satellite and sentby the earth-based satellite terminal according to the TDMAcommunication protocol, relative to their expected, true arrival times;and controlling, with the electronic processor, the satellitetransceiver of the earth-based satellite terminal to transmit third datato the satellite based on the Coarse Timing Advance, the Fine TimingAdvance, the Delta Timing Advance, and the TDMA communication protocol.

Example 23: a time division multiple access (TDMA) communication systemcomprising a satellite; and an earth-based satellite terminal includinga satellite transceiver configured to communicate with the satelliteusing the TDMA communication protocol, a memory, and an electronicprocessor communicatively connected to the memory and the satellitetransceiver, the electronic processor configured to determine a locationof the earth-based satellite terminal, determine a location of thesatellite, estimate a distance between the location of the earth-basedsatellite terminal and the location of the satellite, determine a CoarseTiming Advance based on the distance that is estimated, and control thesatellite transceiver to transmit data to the satellite based on theCoarse Timing Advance and the TDMA communication protocol.

Example 24: the time division multiple access (TDMA) communicationsystem of Example 23, further comprising a satellite hub communicativelycoupled to the satellite and configured to determine a Fine TimingAdvance based on an error in an arrival time of a preamble burst sent bythe earth-based satellite terminal, relative to an expected, truearrival time at the satellite, and transmit the Fine Timing Advance tothe earth-based satellite terminal, wherein the electronic processor isfurther configured to receive the Fine Timing Advance from the satellitehub, and control the satellite transceiver to transmit second data tothe satellite based on the Coarse Timing Advance, the Fine TimingAdvance, and the TDMA communication protocol.

Example 25: the time division multiple access (TDMA) communicationsystem of Example 24, wherein the satellite hub is further configured todetermine a Delta Timing Advance based on errors in actual arrival timesof uplink traffic and control channel bursts at the satellite and sentby the earth-based satellite terminal according to the TDMAcommunication protocol, relative to their expected, true arrival timesat the satellite, and transmit the Delta Timing Advance to theearth-based satellite terminal, and wherein the electronic processor isfurther configured to receive the Delta Timing Advance from thesatellite hub, and control the satellite transceiver to transmit thirddata to the satellite based on the Coarse Timing Advance, the FineTiming Advance, the Delta Timing Advance, and the TDMA communicationprotocol.

Thus, the present disclosure provides, among other things, devices,methods, and systems for uplink synchronization in a time divisionmultiple access (TDMA) satellite network. Various features andadvantages of the present disclosure are set forth in the followingclaims.

What is claimed is:
 1. An earth-based satellite terminal comprising: asatellite transceiver configured to communicate with a satellite using atime division multiple access (TDMA) communication protocol, a memory,and an electronic processor communicatively connected to the memory andthe satellite transceiver, the electronic processor configured todetermine a location of the earth-based satellite terminal, determine alocation of the satellite, estimate a distance between the location ofthe earth-based satellite terminal and the location of the satellite,determine a Coarse Timing Advance based on the distance that isestimated, control the satellite transceiver to transmit data to thesatellite based on the Coarse Timing Advance and the TDMA communicationprotocol, receive a Fine Timing Advance from a satellite hub, whereinthe Fine Timing Advance is determined by the satellite hub based on anerror in an arrival time of a preamble burst sent by the earth-basedsatellite terminal, relative to an expected, true arrival time at thesatellite, and control the satellite transceiver to transmit second datato the satellite based on the Coarse Timing Advance, the Fine TimingAdvance, and the TDMA communication protocol.
 2. The earth-basedsatellite terminal of claim 1, further comprising: an input/outputinterface including a navigation transceiver, wherein, to determine thelocation of the earth-based satellite terminal, the electronic processoris configured to receive navigation information from the navigationtransceiver, a terrestrial wireless navigation system, an inertialnavigation system, or a combination thereof via the input/outputinterface, and determine the location of the earth-based satelliteterminal based on the navigation information.
 3. The earth-basedsatellite terminal of claim 1, wherein, to determine the location of thesatellite, the electronic processor is configured to periodicallyreceive an absolute location of the satellite from the satellite hubassociated with the satellite, the absolute location specifying alatitude, a longitude, and a distance from the center of the Earth withrespect to the satellite.
 4. The earth-based satellite terminal of claim3, wherein the electronic processor is configured to periodicallyreceive the absolute location of the satellite from the satellite hubvia the satellite transceiver and the satellite.
 5. The earth-basedsatellite terminal of claim 3, further comprising: an input/outputinterface including a wireless interface configured to communicate witha terrestrial communication network, wherein the electronic processor isconfigured to periodically receive the absolute location of thesatellite from the satellite hub via the wireless interface and theterrestrial communication network.
 6. The earth-based satellite terminalof claim 1, wherein, to determine the location of the satellite, theelectronic processor is configured to retrieve predefined informationthat is stored in the memory and represents a model of an approximateposition of the satellite, periodically receive a three dimensionalerror vector from the satellite hub, the three dimensional error vectorrepresenting an error between a present location of the satellite and amodeled position of the satellite, and determine the location of thesatellite based on the three dimensional error vector and the modeledposition.
 7. The earth-based satellite terminal of claim 6, where thepredefined information is indicative of a fixed point in space.
 8. Theearth-based satellite terminal of claim 6, where the predefinedinformation is indicative of a set of points in space that form arepetitive function of time.
 9. The earth-based satellite terminal ofclaim 8, where the repetitive function of time represents a figure ofeight.
 10. The earth-based satellite terminal of claim 1, wherein, todetermine the location of the satellite, the electronic processor isconfigured to retrieve predefined information that is stored in thememory and represents a model of an approximate position of thesatellite, and set the location of the satellite based on the model ofthe approximate position of the satellite.
 11. The earth-based satelliteterminal of claim 1, wherein the electronic processor is furtherconfigured to receive a Delta Timing Advance from the satellite hub,wherein the Delta Timing Advance is determined by the satellite hubbased on errors in actual arrival times of uplink traffic and controlchannel bursts at the satellite and sent by the earth-based satelliteterminal according to the TDMA communication protocol, relative to theirexpected, true arrival times at the satellite, and control the satellitetransceiver to transmit third data to the satellite based on the CoarseTiming Advance, the Fine Timing Advance, the Delta Timing Advance, andthe TDMA communication protocol.
 12. A wireless communication methodcomprising: determining, with an electronic processor of an earth-basedsatellite terminal, a location of the earth-based satellite terminal;determining, with the electronic processor, a location of a satellite;estimating, with the electronic processor, a distance between thelocation of the earth-based satellite terminal and the location of thesatellite; determining, with the electronic processor, a Coarse TimingAdvance based on the distance that is estimated; controlling, with theelectronic processor, a satellite transceiver of the earth-basedsatellite terminal to transmit data to the satellite based on the CoarseTiming Advance and a time division multiple access (TDMA) communicationprotocol; receiving, with the electronic processor, a Fine TimingAdvance from a satellite hub, wherein the Fine Timing Advance isdetermined by the satellite hub based on an error in an arrival time ofa preamble burst sent by the earth-based satellite terminal, relative toan expected, true arrival time at the satellite; and controlling, withthe electronic processor, the satellite transceiver of the earth-basedsatellite terminal to transmit second data to the satellite based on theCoarse Timing Advance, the Fine Timing Advance, and the TDMAcommunication protocol.
 13. The wireless communication method of claim12, wherein determining the location of the earth-based satelliteterminal further includes receiving navigation information from anavigation transceiver, a terrestrial wireless navigation system, aninertial navigation system, or a combination thereof via an input/outputinterface of the earth-based satellite terminal; and determining thelocation of the earth-based satellite terminal based on the navigationinformation.
 14. The wireless communication method of claim 12, whereindetermining the location of the earth-based satellite terminal furtherincludes setting a last known location of the earth-based satelliteterminal as the location of the earth-based satellite terminal.
 15. Thewireless communication method of claim 12, wherein determining thelocation of the satellite further includes periodically receiving anabsolute location of the satellite from the satellite hub associatedwith the satellite, the absolute location specifying a latitude, alongitude, and a distance from the center of the Earth with respect tothe satellite.
 16. The wireless communication method of claim 15,wherein periodically receiving the absolute location of the satellitefrom the satellite hub further includes periodically receiving theabsolute location of the satellite from the satellite hub via thesatellite transceiver and the satellite.
 17. The wireless communicationmethod of claim 15, wherein periodically receiving the absolute locationof the satellite from the satellite hub further includes periodicallyreceiving the absolute location of the satellite from the satellite hubvia a wireless interface of the earth-based satellite terminal and aterrestrial communication network.
 18. The wireless communication methodof claim 12, wherein determining the location of the satellite furtherincludes retrieving predefined information that is stored in a memoryand represents a model of an approximate position of the satellite;periodically receiving a three dimensional error vector from thesatellite hub, the three dimensional error vector representing an errorbetween a present location of the satellite and a modeled position ofthe satellite; and determining the location of the satellite based onthe three dimensional error vector and the modeled position.
 19. Thewireless communication method of claim 12, wherein determining thelocation of the satellite further includes retrieving predefinedinformation that is stored in a memory and represents a model of anapproximate position of the satellite; and setting the location of thesatellite based on the model of the approximate position of thesatellite.
 20. A wireless communication method comprising: determining,with an electronic processor of an earth-based satellite terminal, alocation of the earth-based satellite terminal; determining, with theelectronic processor, a location of a satellite; estimating, with theelectronic processor, a distance between the location of the earth-basedsatellite terminal and the location of the satellite; determining, withthe electronic processor, a Coarse Timing Advance based on the distancethat is estimated; receiving, with the electronic processor, a FineTiming Advance from a satellite hub, wherein the Fine Timing Advance isdetermined by the satellite hub based on an error in an arrival time ofa preamble burst sent by the earth-based satellite terminal, relative toan expected, true arrival time at the satellite; and controlling, withthe electronic processor, a satellite transceiver of the earth-basedsatellite terminal to transmit data to the satellite based on the CoarseTiming Advance, the Fine Timing Advance, and a time division multipleaccess (TDMA) communication protocol.
 21. The wireless communicationmethod of claim 20, further comprising: receiving, with the electronicprocessor, a Delta Timing Advance from the satellite hub, wherein theDelta Timing Advance is determined by the satellite hub based on errorsin actual arrival times of uplink traffic and control channel bursts atthe satellite and sent by the earth-based satellite terminal accordingto the TDMA communication protocol, relative to their expected, truearrival times; and controlling, with the electronic processor, thesatellite transceiver of the earth-based satellite terminal to transmitthird data to the satellite based on the Coarse Timing Advance, the FineTiming Advance, the Delta Timing Advance, and the TDMA communicationprotocol.
 22. A time division multiple access (TDMA) communicationsystem comprising: a satellite; and an earth-based satellite terminalincluding a satellite transceiver configured to communicate with thesatellite using the TDMA communication protocol, a memory, and anelectronic processor communicatively connected to the memory and thesatellite transceiver, the electronic processor configured to determinea location of the earth-based satellite terminal, determine a locationof the satellite, estimate a distance between the location of theearth-based satellite terminal and the location of the satellite,determine a Coarse Timing Advance based on the distance that isestimated, and control the satellite transceiver to transmit data to thesatellite based on the Coarse Timing Advance and a time divisionmultiple access (TDMA) communication protocol; and a satellite hubcommunicatively coupled to the satellite and configured to determine aFine Timing Advance based on an error in an arrival time of a preambleburst sent by the earth-based satellite terminal, relative to anexpected, true arrival time at the satellite, and transmit the FineTiming Advance to the earth-based satellite terminal, wherein theelectronic processor is further configured to receive the Fine TimingAdvance from the satellite hub, and control the satellite transceiver totransmit second data to the satellite based on the Coarse TimingAdvance, the Fine Timing Advance, and the TDMA communication protocol.23. The time division multiple access (TDMA) communication system ofclaim 22, wherein the satellite hub is further configured to determine aDelta Timing Advance based on errors in actual arrival times of uplinktraffic and control channel bursts at the satellite and sent by theearth-based satellite terminal according to the TDMA communicationprotocol, relative to their expected, true arrival times at thesatellite, and transmit the Delta Timing Advance to the earth-basedsatellite terminal, and wherein the electronic processor is furtherconfigured to receive the Delta Timing Advance from the satellite hub,and control the satellite transceiver to transmit third data to thesatellite based on the Coarse Timing Advance, the Fine Timing Advance,the Delta Timing Advance, and the TDMA communication protocol.